1. Field of the Invention
The present invention relates to a toner and a developer used for forming an image in an electrostatic copying process, such as for a copier, a facsimile, and a printer. The present invention further relates to an image developing apparatus and an image forming apparatus in which the developer is used.
2. Description of the Related Art
An image forming process according to an electrophotographic process comprises a charging step for giving an electric charge to the surface of an photoconductor, which is a latent image carrier, by means of an electric discharge; an exposing step for exposing the charged surface of the photoconductor to form a latent electrostatic image; a developing step for supplying a toner to the latent electrostatic image formed on the surface of the photoconductor to develop a toner image; a transferring step for transferring the toner image on the surface of the photoconductor onto the surface of a transfer material; a fixing step for fixing the toner image on the surface of the transfer material; and a cleaning step for eliminating the residual toner remaining on the surface of the photoconductor after the transferring. In recent years, there has been increasingly demands for higher quality images, and in particular, to realize forming a high-precision color image, toner's smaller sizing (namely smaller diameter of toner particle) and toner's conglobation (rounded spherical form) are under way. Toner's smaller sizing enables excellent dot-reproductivity, and toner's conglobation makes it possible to improve developing properties and transferring properties. Since it is very difficult to manufacture such a smaller-particle-sized and conglobated toner by a conventional kneading and grinding method, there is a growing adoption of a polymerized toner manufactured by a suspension polymerization method, an emulsion polymerization method, and a dispersion polymerization method.
However, if a toner particle diameter is sized down up to a few micrometers or less, non-electrostatic adherence, such as, van der Waals force or the like which works on between the toner and a photoconductor increases in proportion to its weight empty, and therefore, mold-release properties become degraded, which affects transferring properties and cleaningability, and the like.
On the other hand, since a toner which is conglobated and formed in a shape close to a perfect sphere has a lower adherence with photoconductors or the like than that of a toner in indefinite (undetermined) forms obtained by a kneading and grinding method, a higher transfer rate can be obtained because the conglobated toner has excellent mold-release properties. Besides, the conglobated toner makes an image transfer true to a latent image along the line of electric force, because the toner particles also have a low adherence each other and therefore the toner is susceptible to the line of electric force. However, when a transfer material is released from a photoconductor, a high electric field is induced between the photoconductor and the transfer material (burst phenomenon), which causes a problem that the toner on the transfer material and the photoconductor is scattered and toner dust occurs on the transfer material.
If a toner formed in a shape close to a perfect sphere is in a condition where the toner just has been transferred onto a transferring paper but not yet fixed, there is a problem that the toner is liable to roll by contact with a fixing member in a fixing step, which causes a disordered image, since such toner particles has a low adherence each to each, as stated above.
Further, a toner formed in a shape close to a perfect sphere has a problem that it is hard to be cleaned by blade cleaning which has been used so far. This is because a conglobated toner is liable to roll on the surface of a photoconductor and the toner slips through a clearance between the photoconductor and a cleaning blade.
For the reasons mentioned above, it becomes a new challenge to control a surface shape of a toner so as to be able to properly give an adherence between a toner and a photoconductor or an adherence among toner particles while providing a toner design in consideration of toner's smaller sizing and conglobation. There have been various proposals presented so far for controlling a toner's surface shape of its smaller sizing and conglobation particularly with a view to improving cleaningability. For example, there is a proposal in which through the use of SF-1 (shape factor-1) which is an indicator representing the level of roundness (sphericity) of a toner particle and SF-2 (shape factor-2) which is an indicator representing the level of concave and convex of a toner particle to represent a toner shape, improvements in cleaningability are performed by defining one shape factor of SF-1 or SF-2 or both shape factors to control a toner's shape
(for example, see Japanese Patent Application Laid-Open (JP-A) Nos. 2000-122347, 2000-267331, 2001-312191, 2002-23408, 2002-311775, and 09-179411).
However, there may be cases where with improved cleaningability, it becomes difficult to make a toner have such a shape that a balance between favorable transferring properties and fixability can be achieved. There is no toner presented in which toner's surface shape is considered and examined from the perspective of improvements in not only cleaningability but also transferring properties and fixability.